Showtime, there is no point being angry with yourself now, we could all have done things differently at one stage or another if only we knew... Try to just concentrate on the present and how best to deal with it.
First of all, move your savings to another bank. Then try to negotiate the extension of the interest-only period on your investment property. If both your properties are in negative equity, the bank is hopefully not very likely to push for repossession now, but it is best to avoid falling into arrears, as arrears will build up over time. And when property prices recover (they will some day, even if that day is a long way off) or when you pay down a good chunk of your main mortgage - then the bank might push for repossession to recoup the arrears. So it is best to obtain agreement from the bank for reduced payments or interest-only.
Also if you fall into arrears it will damage your credit rating. The credit rating might not be much use to you now, but if you manage to pay down negative equity on your main mortgage, you might be able to remortgage to a more competitive rate with another bank.
I'd advise you to keep a spending diary for a month or two, just to make sure that there really is nothing you can scale back on. A lot of people think that they are being frugal but there usually are still some expenses that can be scaled back.
If you can't cut back much more on expenses, how about getting any extra income? Rent-a-room, for example, and/or extra evening/weekend work, grinds, babysitting - anything that will earn you a bit of extra cash? Also selling unwanted stuff at ebay or car boot sales? I know selling a bit of unwanted stuff won't solve all your financial problems, but every little helps...
Another thing - if your investment property is considerable cheaper than the PPR, would it be possible or worthwhile for you to move there and rent your main property out instead, thereby getting more rent?